Chitin is a natural high molecular weight polymer widely found in nature. It is the main component of insect and crustacean cuticle, and is also part of the cell walls of some fungi and other organisms. Chitin is generally extracted from its natural sources by treatments with strong acid (to remove calcium deposits where required) and strong alkali (to remove proteinaceous residue). Chitin is insoluble under typical aqueous conditions and is considered to be a relatively intractable polymer (difficult to process). Dissolution of chitin to enable direct processing into fibers or other forms requires the use of unattractive solvent systems that are generally corrosive and toxic.
Chitosan is produced at the industrial level by hydrolytic deacetylation of chitin. Chitin and chitosan are part of the glycosaminoglycan family of polymers. Chitosan is typically derived from chitin by deacetylation in the presence of alkali. Chitosan is a generic term used to describe linear polysaccharides which are composed of glucosamine and N-acetyl glucosamine residues joined by β-(1-4) glycosidic linkages (typically the number of glucosamines≧N-acetyl glucosamines) and whose composition is soluble in dilute aqueous acid. The chitosan family encompasses poly-β-(1-4)-N-acetyl-glucosamine and poly-β-(1-4)-N-glucosamine with the acetyl residue fraction and its motif decoration (either random or block) affecting chitosan chemistry. The 2-carbon amino group on the glucosamine ring in chitosan allows for protonation, and hence solubilization of chitosan in water (pKa≈6.5) (Roberts). This allows the ready processing of chitosan into fibers, films, and other forms, as well as the ability to prepare high purity chitosan for biomedical use.
Depending on original biological material sourcing and control of processing of chitin to chitosan, poly-N-acetyl-glucosamine compounds exhibit widely differing physical and chemical properties which are processable from aqueous solution. These differences are due to chitosan's varying molecular weights, varying degrees of acetylation, the presence of contaminants such as covalently bound, species-specific proteins, single amino acid and inorganic contaminants, etc.
Much attention has been paid to chitosan as a functional polymer because several distinctive biomedical properties such as non-toxicity, biocompatibility and biodegradability have been reported. Indeed, chitosan is widely regarded as being a non-toxic, biologically compatible polymer. (Kean, T. et al., 2005, Adv. Drug Deliv. Rev. 62:3-11 (“Kean”); Ren, D. et al., 2005, Carbohydrate Res. 340(15):2403-10 (“Ren”)).
The potential safe, biocompatible, and bioabsorbable use of chitosan makes it an attractive natural material for use in biomedical implants. Further, deacetylated and partially deacetylated chitin preparations exhibit potentially beneficial chemical properties, such as high reactivity, dense cationic charges, powerful metal chelating capacity, the ability to covalently attach proteins, and solubility in many aqueous solvents. Also, it is conventionally understood that chitosan adheres to living tissue, acts as a haemostatic agent, promotes rapid healing, and has antibacterial properties.
These chemical and biological properties are now beginning to prove useful in many medical applications. Although chitosan compositions are being used increasingly in the United States, Europe, and Asia in external medical applications, such as wound composition products, sponges, powdered haemostatic agents, and antimicrobial gels, biocompatible and bioabsorbable chitosan compositions are yet to be approved for internal surgical use.
Chitosan has not gained usage as a biocompatible and bioabsorbable biomedical implant material, at least in part, because chitosan comprises a large group of structurally different chemical entities and its biodegradation properties are driven by multiple co-dependent factors that render composition design unpredictable. Various physicochemical characteristics of chitosan, such as molecular weight, degree of deacetylation, and distribution of acetamide groups in the chitosan molecule, influence chitosan function and bioabsorbability. (Kofuji, K. et al., 2005, Eur. Polymer J. 41:2784-2791 (“Kofuji”)). Of these characteristics, molecular weight and the degree of N-deacetylation (DDA) are believed to be the two most important determinants of the bioabsorbability properties of chitosan. (Ren; Kean).
The in vivo degradation of chitosan is not fully understood but it is believed to occur by enzymatic cleavage of the polymer chain. (Kean; Ren). Lysozyme is the most prominent of the chitosan degrading enzymes in humans, however there are various other chitanases generally found in animals, plants, and microbes. (Kean). The degradation behavior of chitosan plays a crucial role in biocompatible material performance. The degradation kinetics may affect many cellular processes, including cell growth, tissue regeneration, and host response. (Ren). Investigations regarding the degradation of chitosan by lysozyme indicate that the DDA of chitosan is one of the key factors controlling the degradation of chitosan. (Id.). Also, it has been noted that N-substitution may affect enzymatic degradation. (Kean).
The rate of biodegradation and bioabsorption in vivo is also subject to the competing process of foreign body encapsulation (fibrous capsule formation) which may ultimately wall-off the bioabsorbing composition if the rate of its bioabsorption is sufficiently slow and the foreign body elicits a moderate inflammatory response to promote an enhanced rate of encapsulation. Such encapsulation is undesirable for an intended bioabsorbable composition since it can extend the residence time of the composition in vivo potentially from months to years. A reduced rate of encapsulation combined with timely clearing and removal of the foreign body is desired since protracted residence time can result in the adverse events of vascular and/or neural impingement as well as promote infection.
A prerequisite for effective scission of chitosan by lysozyme is that there are regular groupings of at least three consecutive N-acetyl glucosamine monomers in the polysaccharide chain (Aiba), i.e., the DDA of chitosan is sufficiently low (<70% DDA) with the necessary N-acetyl motif structure to enable systematic enzymatic cleavage. Generally, the more acetyl groups on the chitosan, the faster its degradation rate. (Tomihata, K. et al., 1997, Biomaterials 18:567-575 (“Tomihata”)).
The water soluble range for chitosan above pH 6.5, which is between 45% and 55% DDA (Roberts 1992), often causes confusion in the determination of absolute rates of scission and of bioabsorption since water soluble chitosan will appear to bioabsorb more quickly when in fact it has only dissolved. (Freier, T. et al., 2005 Biomaterials, 26 (29):5872-8 (“Freier”)).
As a general matter in addition to its DDA, other chitosan molecule characteristics such as its molecular weight, viscosity, solubility, and distribution of acetamide groups affect chitosan's bioabsorption properties.
Also, as indicated previously, biomaterial biocompatibility plays an important role in bioabsorption. Biomaterials which are biodegradable by enzymatic, hydrolytic or oxidative pathways, but which only slightly elevate the local biomaterial inflammatory response, will bioabsorb at a slower rate than biodegradable biomaterials that moderately elevate the same response. Interestingly, chitosan at high DDA is shown to have very good biocompatibility, with reported biocompatibility declining as DDA is reduced. (Tomihata).
It is conventionally understood that chitosan bioabsorption and biodegradation requires chitosan with DDA less than 70% and more than 40% DDA if the poly-β-(1-4) N-acetyl glucosamine is to still be considered chitosan (soluble in dilute aqueous solution). Pure chitin (DDA near 0.0) has shown to be bioabsorbable. (Tomihata). Chitosan compositions at about 70% DDA and higher demonstrate minimal biodegradation due, at least in part, to lack of acetyl groups to prompt enzymatic cleavage and/or lack of solubility. (See Freier, T. et al., 2005 Biomaterials, 26 (29):5872-8 (“Freier”)).
It has been found that within a week of implantation that these higher DDA chitosans, while showing very good biocompatibility, begin to experience encapsulation. (Vandevord). As such, chitosan compositions near 70% DDA and higher, with their slow rate of biodegradation and encapsulation, may never fully resorb in vivo, and may produce undesirable encapsulation.
It is reported in the literature that chitosan having below a 70% DDA is demonstrated as biocompatible and bioabsorbable and is proposed as safe for biomedical use. Only chitosan compositions having a DDA of between about 40-70%, however, have been demonstrated to bioabsorb in vivo with the definition of the poly-β-(1-4) N-acetyl glucosamine being chitin or chitosan at the lower DDA being dependent, as per Roberts, on its water solubility at or below pH 6.5. The biocompatibility of these bioresorbable chitosan compositions, although less than high DDA chitosan, has been reported to warrant further investigation. The number of reported in vivo studies of bioabsorption of chitosan with actual bioabsorption occurring, however, is very low. (See e.g., Tomihata). The majority of other studies purporting to study chitosan bioresorption use only in vitro enzymatic conditions (generally lysozyme). As shown, lysozyme solution allows for analysis of the relative susceptibly of chitosan to biodegrade in vivo, however, it cannot account for absorption and biodegradation effects associated with biomaterial biocompatibility and the biocompatibility of the biodegradation products.
The biocompatibility and bioabsorption of chitosan compositions with DDAs lower than 40% have not been widely investigated. This may be, in part, due to the fact that achieving a chitosan with a lower DDA can be difficult. (Ren). Also, lowering the DDA of chitosan below 40% to achieve faster rates of biodegradation is frustrated by the fact that chitosan having a DDA less than 40% should make the chitosan insoluble in aqueous solution below pH 6.5 and hence not chitosan as per the Roberts definition. (See e.g., Ren; Xu, J. et al., 1996, Macromolecules 29:3436-3440 (“Xu”); Freier). Further, chitosans having DDAs below 50% are not typically commercially available.
Nonetheless, to the extent that lowering the DDA of chitosan may beneficially serve to increase its rate of biodegradation, the conventional wisdom is that too fast a rate of biodegradation may be undesirable as it is well-known that the more rapidly biomaterials biodegrade, the more likely they are to elicit an acute inflammation reaction due to a significantly large production of low-molecular-weight compounds within a short time. (Tomihata).
Additionally, preparing compositions to include chitosan having the water soluble range of 45%-55% DDA will cause undesirable swelling and fluid absorption by the compositions that may cause undesirable and unpredictable fluctuations in implant size and performance during bioabsorption.
As detailed below, the inventors of the present invention have surprisingly discovered that, contrary to conventional wisdom and industry practice, compositions comprising derivatized non-crosslinked chitosan compositions with a DDA range between 40% and about 70% are toxic when implanted, biodegraded, and bioabsorbed. The present inventors have also surprisingly discovered that biocompatible, non-toxic and bioabsorbable biomedical chitosan compositions with a DDA range of between about 15% and 40% can be prepared that, upon implant, are at least 85% bioabsorbed within about 90 days or less. Accordingly, the present inventors have not only overcome widely held misconceptions by those skilled in the art regarding the biocompatibility and bioabsorbability of compositions comprising chitosan with a DDA range between 40% and about 70%, but they have achieved 1) the surprising identification of a biocompatible and bioabsorbable chitosan DDA range of between about 15% and 40%, 2) methods of making the inventive compositions comprising derivatized non-crosslinked chitosan using chitosan having a DDA range of between about 15% and 40%, and 3) developed a modified acute systemic toxicity test to ensure that the compositions of the present invention, when implanted, do not produce toxic biodegradation species giving rise to an elevated IL-1β cytokine response.